1. Field of the Invention
The present invention relates to a position measuring device for measuring the relative positions of two objects. The invention particularly relates to a position measuring device in which the graduation of a graduation carrier connected to one of the objects is scanned by a scanning unit which is coupled in an articulated manner through a coupling which is stiff in measuring direction to the other object or to an engagement member mounted on the other object. The scanning unit is movable on an auxiliary guide means which is independent of the guide means of the two objects. The coupling includes two coupling members which contact each other.
2. Description of the Related Art
Position measuring devices of the type described above are particularly used in processing machines for measuring the relative position of a tool relative to a workpiece to be processed.
In measuring devices with a scanning unit which is guided on an auxiliary guide means which is independent of the guide means of the two objects in the form of machine parts, an articulated coupling to the corresponding object or an engagement member arranged on the corresponding object is indispensible.
German patent 26 11 459 describes a position-measuring device in which a scanning unit for scanning the graduation of a graduation carrier is movable on an auxiliary guide means which is independent of the guide means of the two objects whose relative positions are to be measured and is coupled in an articulated manner to an engagement member through a coupling which is stiff in measuring direction. The first coupling part of this coupling is a plane surface on the scanning unit which is in continuous contact by means of a spring with a spherical surface of the second coupling part on the engagement member.
However, in this known type of coupling, parallel displacements and angular errors between the guide means of the two objects and the auxiliary guide means cause relative movements between the plane surface and the spherical surface and the resulting sliding friction leads to wear of the two coupling parts and, thus, to measuring inaccuracies which can no longer be tolerated in high-precision position measuring devices.